1. Field of the Invention
The present invention relates to an improved reactive power compensator effective in stabilizing voltage of a system.
2. Description of Related Art
From the viewpoint of stably supplying electric power from a generator or a transformer (as well as a power plant or a substation, hereinafter collectively referred to as a power unit) to a load, it is extremely important to keep the voltage supplied to the load within a predetermined range. It is a matter of course that any output voltage of the power unit is controllable in most cases. However, there may be a voltage drop due to distribution line and transformer provided on the midway. In particular, when a system is operated in cooperation with another system, it is not always possible to regulate the voltage from the viewpoint of the cooperation with another system, and consequently, the voltage supplied to the load fluctuates in some case.
Hitherto, equipment for regulating the voltage of the system has been used in the form of being interposed midway in the system (hereinafter referred to as voltage regulator for convenience of explanation) in order to minimize such voltage fluctuation. Typical equipment capable of discontinuously changing the voltage (regulating the voltage to a discrete value with a difference between one value and another by a predetermined width) such as tap-changing transformer has been already proposed as such equipment.
FIG. 7 is a flow diagram for explaining constitution and operation of a system including such a conventional voltage regulator In the drawing, reference numeral 15 is a power unit of the system. Specifically, the power unit can be a single generator or an output transformer of a substation. Numeral 16 is a system impedance of the power unit 15, a distribution line connected to the power unit 15 etc., and the impedance is shown as lumped impedance for convenience of explanation. Numeral 17 is a tap-changing transformer with a tap control 17a, which is an example of the voltage regulator, added on a secondary side. Numeral 20 is a reactive power compensator connected to the secondary side of the tap-changing transformer 17, and numeral 21 is a load.
On the supposition that:
Vo is a voltage of the power unit 15,
Is is a power supply current flowing through the impedance 16,
Vs is a voltage on the primary side of the tap-changing transformer 17,
Vt is a voltage on the secondary side of the tap-changing transformer 17 (or a voltage of the load 21 in some cases),
1 to k is a transformation ratio (when a reference voltage is outputted) of the tap-changing transformer 17,
I is an electric current flowing through the load 3, and
Iq is a compensation current of the reactive power compensator 20;
the electric current Is of the power supply flowing through the impedance 16 is a value obtained by subtracting the compensation current Iq of the reactive power compensator 20 from the load current I flowing through the load 21 and multiplying the remainder value by the tap ratio k of the tap-changing transformer 17.
That is, Is=(Ixe2x88x92Iq)xc2x7kxe2x80x83xe2x80x83(1)
The load voltage Vt is obtained as follows:
Vt=(Voxe2x88x92Xxc2x7Is)xc2x7k
xe2x80x83={Voxe2x88x92k(Ixe2x88x92Iq)}xc2x7kxe2x80x83xe2x80x83(2)
Now, voltage stabilization operation of the system in FIG. 7 is hereinafter described. To simplify the explanation, in the description, it is supposed that the load 21 is of a complete inductive load.
For better understanding, the systematic diagram in FIG. 7 is shown in FIG. 8 in the form of a block diagram showing the relation between the load terminal voltage Vt and the load current I etc. The operation of the tap control 17a is publicly known and detailed description of the operation is omitted herein. Fundamentally, as shown in FIG. 9, the operation includes decrease in voltage when the load voltage Vt is exceeding a voltage Vmax that was set at a value higher than a reference voltage Vref, or increase in voltage when the load voltage Vt is lower than Vmin set on the lower side. At this time, the tap voltage of the tap-changing transformer 17 is preliminarily set so that both of the value after the voltage increase and the value after the voltage decrease may come within the range of Vmax to Vmin. Vmax and Vmin are referred to as boundary voltage, and the zone between Vmax and Vmin is referred to as dead zone.
FIG. 10 is a characteristic graph showing the relation between the load current I and the terminal voltage Vt of the load 21. For better understanding, first, operation of the system is hereinafter described on the supposition that the reactive power compensator 20 is not connected (i.e., Iq=0).
It is herein supposed that the reference voltage Vref of the tap change operation of the tap control 17a is equal to the voltage Vo of the power supply 15 and a point A in FIG. 10 (when the load current I=0 and Vt=Vref) is an initial condition. When the load current I is 0, the load voltage Vt is Vo equal to the foregoing reference value of the tap change operation. Therefore, when increasing the load current I, the primary side voltage Vs of the tap-changing transformer 17 drops from the power supply voltage Vo by Xxc2x7Is, and the load voltage Vt also drops on the right side region of the point A (on the side region where the load current I increases) as shown in FIG. 10. At this point, if k=1, the dropped voltage is obtained based on the foregoing expression (2) as follows:
Vt=Voxe2x88x92Xxc2x7Isxe2x80x83xe2x80x83(3)
Then, if Vt continues to further drop to be lower than the voltage Vmin which is lower than Vref by the width VD of the dead zone possessed by the tap control 17a as its characteristic (for example, when a quantity S obtained by time integration of a quantity deviated from Vmin comes to reach a predetermined quantity Sref, as described later in detail), the tap control 17a changes the tap position of the tap-changing transformer 17 by one stage toward the voltage increase side. Thus, as indicated by the point B in FIG. 10, Vt increases within a range not reaching Vmax.
When I increases further, Vt drops to reach Vmin again, the tap position is changed again, and Vt increases again. Such an operation will be repeated to the limit of the tap position.
Though not describing in detail, in the case that the load current I flows in the reverse direction (in power regeneration direction) and the load voltage Vt increases, the fundamental operation is the same. There is a difference only in the aspect that the operating voltage of the tap control 17a is changed to the Vmax side and the voltage comes to drop at the operating point. In this case, the voltage is dropped within the range not reaching Vmin as a matter of course.
Described hereinafter is the case in which the reactive power compensator 20 is connected. To simplify the explanation, in the following description, it is supposed that I is 0 and Vt=Vref=Vo under the initial condition in the same manner as in the foregoing description of the case without the reactive power compensator 20.
When K=1, the power supply current Is becomes (Ixe2x88x92Iq), and therefore Is is small as compared with the case without the reactive power compensator 20 by a compensation current of the reactive power compensator 20. Accordingly, the voltage drop (Xxc2x7Is) caused by the systematic impedance X becomes small, and drop in Vs is not so large, and the drop of Vt is also small as much. In the case that the reactive power compensator 20 is used in order to stabilize the voltage, this principle is used to keep the load voltage Vt.
In general, as shown in FIG. 11, the output current Iq of the reactive power compensator 20 is a value obtained by multiplying a difference between an operating reference voltage Vtref set in the reactive power compensator 20 and the load voltage Vt by a gain G. Under the initial condition, supposing that the voltage command value Vtref of the reactive power compensator 20 is equal to Vref, there is no difference between Vtref and Vt, and therefore Iq is 0. Iqlimit indicated in the drawing is a current limit value (limiter) generally set for the purpose of protecting the apparatus, and the foregoing description is reasonable within this range.
When increasing I, Vt drops. Then, the current Iq that is a result of multiplying the difference between Vtref and Vt by G is supplied to the system.
Vt is, as shown in the expression (2),
Vt=k{Voxe2x88x92Xxc2x7kxc2x7(Ixe2x88x92Iq)}
where:
Iq=(Vtrefxe2x88x92Vt)xc2x7G
k=1
Vtref=Vref=Vo
Substituting the above expressions for (2) and arranging them,                                                         Vt              =                              Vo                -                                  X                  ·                                      {                                          I                      -                      Iq                                        }                                                                                                                          =                              Vo                -                                  X                  ·                  I                                +                                  X                  ·                  Iq                                                                                                        =                              Vo                -                                  X                  ·                  I                                +                                  X                  ·                                      G                    ⁡                                          (                                              Vtref                        -                        Vt                                            )                                                                                                                                                              Vt                +                                  Vt                  ·                  X                  ·                  G                                            =                              Vo                -                                  X                  ·                  I                                +                                  X                  ·                  G                  ·                  Vtref                                                                                                                        Vt                ⁡                                  (                                      1                    +                    XG                                    )                                            =                              Vo                -                                  X                  ·                  I                                +                                  X                  ·                  G                  ·                  Vo                                                                                                        Vt              =                                                {                                                            Vo                      ⁡                                              (                                                  1                          +                          XG                                                )                                                              -                    XI                                    }                                /                                  (                                      1                    +                    XG                                    )                                                                                                        Vt              =                              Vo                -                                  X                  ·                                      I                    /                                          (                                              1                        +                        XG                                            )                                                                                                                              (        4        )            
As compared with the foregoing expression (3) of voltage in the case without the reactive power compensator 20, the drop ratio of Vt to the increase in I is reduced to 1/(1+XG). FIG. 12 shows variation in the load voltage Vt with respect to variation in the load current I, and the outputting manner of the output current of the reactive power compensator 20. The left end of this graph indicates a limit of the tap change.
In general, it is possible to set a large value for G, and therefore within the range of control of the reactive power compensation (within the range of change in Iq shown in FIG. 12), the drop of Vt is extremely small (the Vt characteristic declines slightly in FIG. 12). However, as the reactive power compensator 20 is comprised of power semiconductors in most cases, Iq cannot be increased exceeding a limit value Iqmax fixed in accordance with a capacity of the apparatus. Once reaching Iqmax (point C in FIG. 12), Iq is fixed to Iqmax and therefore
Vt=Voxe2x88x92Xxc2x7I+Xxc2x7Iqmaxxe2x80x83xe2x80x83(5)
At this time, values of Vo and Xxc2x7Iqmax are fixed, and therefore Vt gradually drops with the same inclination in the expression (3) showing the case without the reactive power compensator 20. When Vt comes to less than the operating value Vmin on the increase side of the tap control of the transformer, the tap position is changed, Vt increases, and Iq decreases. When further increasing I, Vt drops again and Iq comes to increase.
In order that the reactive power compensator 20 may operate as described above, it is necessary that a parameter thereof is set correctly. However, the tap-changing transformer 17 and the tap control 17a have various characteristics depending upon their capacities and the state of the system at an insertion point, and therefore the parameter cannot be set correctly until the position for connecting the reactive power compensator 20 in the system is fixed and the characteristics of the connected tap control 17a etc. are made clear. After all, correct setting of parameter must be carried out at the working site, hence a problem exists in that the setting requires much time.
Basically, the tap change is slow in response and can cope with only slow fluctuation in system voltage, while the reactive power controller is quick in response and can cope with sharp fluctuation in system voltage, and therefore the setting needs to be arranged making the best use of such characteristics. However, in the case of the characteristic shown in FIG. 12, it is considered that, in most cases, the reactive power compensator 20 outputs a reactive power to regulate the voltage and is operated under the maximum value of the output of the reactive power compensator (i.e., from the point C to a point D in FIG. 12), before tap control is performed with respect to the fluctuation in the system voltage Vt. In such a case, if any condition of the system varies suddenly and the voltage fluctuation takes place too quickly to respond by the tap change control, it is not possible to urgently generate reactive power to stabilize the system voltage because the output of the reactive power compensator 20 is already at maximum and cannot be increased any more. To cope with this, for example, in the xe2x80x9cStatic Var Compensator (STATCOM) voltage Control Method in Consideration of Transformer Tapxe2x80x9d made public at the Annual National Convention 2000 of the Institute of Electrical Engineers of Japan (IEEJ), a gain G of STATCOM being a type of reactive power compensator is changed into two stages. That is, G is changed to G1, which is a low gain, when voltage deviation is small, and G is changed to G2, which is a high gain, when the voltage deviation increases to a certain degree.
In this method, when the voltage deviation remains within a certain range, the output of the reactive power compensator 20 never reaches the maximum value, and it is said possible to cope with a case in which it is necessary to further abruptly change the reactive power. However, it is not always defined that the value of G should be set within a specific voltage deviation range and at a specific level, and therefore the setting must be performed on a trial and error basis, thus a problem still exists in that the setting requires much time.
In the conventional reactive power compensator of above-described arrangement, a problem exists in that the setting is impossible unless the characteristics of the connected tap-changing transformer etc. are clear.
It is certain that a method has been proposed for covering fluctuation in the load current I to a certain extent within a range of tap change and also covering fluctuation too transient and large to respond by the tap change. But, it is not always defined how the gain should be set, and therefore the setting must be performed on a trial and error basis, thus a problem still exists in that the setting requires much time.
The present invention has been made to solve the above-discussed problems and has an object of providing a reactive power compensator in which a method of setting a parameter of the reactive power compensator is defined, a time required for setting the parameter is shortened, and the setting is automatized.
A reactive power compensator according to the invention has a computing unit that is connected to an output side of a voltage regulator connected to a power system. And in the voltage regulater a voltage to be supplied to a load deviates from a dead zone having boundary voltages each above and below a predetermined reference value, the voltage is regulated to be predetermined discrete values each different by a predetermined width, and calculates an electric current by multiplying a difference between the voltage load and the reference value by a gain calculated on the basis of the boundary voltages or the predetermined discrete value.
And the calculated electric current is inputted to and outputted from the power system. And the gain includes at least two stages of gains (a first and a second gain).
The first gain is used at the time when the voltage of the system is within the dead zone or outside the dead zone and in the vicinity of one of the boundary voltages.
The second gain is used at the time when the voltage of the system is outside the dead zone and is not in the vicinity of the boundary voltages.
The second gain is larger than the first gain and smaller than twenty times the first gain.
As a result, the output is restrained to be within a certain level capable of securing a control margin.
Using the secured control margin to overcome the sudden fluctuation in the system voltage makes it possible to obtain a broader voltage stabilization characteristic.